When transmitting data across a medium, wired or wireless, it is generally desirable to increase the amount of data that is sent at a given time to allow for a higher data rate for the underlying system. In a single-antenna, wireless transmission system, typically a single stream of data is transmitted.
However, using a multiple antenna system, multiple streams can be transmitted and received using the same time and frequency resources, and correspondingly more information can be sent over the available bandwidth. As shown below, it is possible to use multiple antenna systems and multi-input, multi-output (MIMO) systems interchangeably. In MIMO systems, there are a number of possible signal paths through the transmission medium equal to the number of transmitting antennas multiplied by the number of receiving antennas.
FIG. 1 is a block diagram illustrating a conventional MIMO transmitter/receiver pair. As shown in FIG. 1, a transmitter includes a MIMO modulator 110, an antenna 130, and an antenna 135, while a receiver includes a MIMO demodulator 120, an antenna 140, and an antenna 145. The MIMO modulator receives transmit signals and modulates them for transmission over antenna 130 and antenna 135. These signals are received at antenna 140 and antenna 145, and the received signals are demodulated at the MIMO demodulator 120 to extract a received signal.
Because there are two transmitting antennas 130 and 135, and two receiving antennas 140 and 145, there are four possible signal paths through the transmission channel. There is a path from the antenna 130 to the antenna 140; a path from the antenna 130 to the antenna 145; a path from the antenna 135 to the antenna 140; and a path from the antenna 135 to the antenna 145. MIMO systems with a larger number of transmitting or receiving antennas would have a correspondingly larger number of possible signal paths.
The MIMO receiver antennas 140 and 145 thus each receive a copy of each signal transmitted from all of the transmitting antennas. But each copy of a transmitted signal from a given transmitter antenna passes through a different portion of the transmission channel, and is multiplied by a different channel coefficient.
In particular, h11 represents a channel portion from a transmitter 130 antenna to a receiver antenna 140; h21 represents a channel portion from the transmitter antenna 130 to a receiver antenna 145; h12 represents a channel portion from the transmitter antenna 135 to the receiver antenna 140; and h22 represents a channel portion from the transmitter antenna 135 to the receiver antenna 145. A general representation H of the transmission channel, as it pertains to the MIMO transmitter and receiver, can then be written as a matrix of the individual channel coefficients h11, h12, h21, and h22. In other words,
                              H          22                =                              [                                                                                h                    11                                                                                        h                    12                                                                                                                    h                    21                                                                                        h                    22                                                                        ]                    .                                    (        1        )            
More generically a channel HMN can be written as an (N×M) matrix where M is the number of transmit antennas and N is the number of receiver antennas. Thus, a (3×2) MIMO system will have a channel represented by a channel matrix H32:
                                          H            32                    =                      [                                                                                h                    11                                                                                        h                    12                                                                                        h                    13                                                                                                                    h                    21                                                                                        h                    22                                                                                        h                    23                                                                        ]                          ,                            (        2        )            
while a (4×2) MIMO system will have a channel represented by a channel matrix H42:
                              H          42                =                              [                                                                                h                    11                                                                                        h                    12                                                                                        h                    13                                                                                        h                    14                                                                                                                    h                    21                                                                                        h                    22                                                                                        h                    23                                                                                        h                    24                                                                        ]                    .                                    (        3        )            
A transmitter with a single antenna can only modulate data in time. In other words, it can only send different portions of data through the antenna at different times. A MIMO transmitter, in contrast, can modulate the data in both space and time. In other words, in addition to sending different portions of data at different times, the MIMO transmitter can also send different portions of data over the different antennas as the same time. This allows a MIMO transmitter to send a larger amount of data in the same time period as compared to a single antenna system.
The MIMO receiver receives the various signals at its receiver antennas from across the various individual channel portions, and its MIMO demodulator 120 uses MIMO signal processing to extract the different portions of data that were sent.
In addition, rather than increasing data rate, a MIMO system could also use be used to increase the range of the transmission system by increasing the level of redundancy, error correction, or the like.
However, because a wireless channel H is a fading channel (i.e., signals passing through it reflect off of objects and take multiple paths to the receivers), each transmitted signal experiences fading, causing a reconstructed signal at the MIMO receiver to itself experience fading. This fading can require an increased cost and complexity for the MIMO receiver to properly process the faded signal, and increased time for such back end processing.